Process and system for removing heavies from diluent recycled to a slurry polymerization reactor

ABSTRACT

A recycling and recovery system and process comprising a flash gas separator that receives a slurry comprising liquid medium and solid polymer particles. The flash gas separator separates the diluent from the solid polymer particles as a vapor stream comprising at least diluent and heavies. A line receives the vapor stream from the flash gas separator. The line leads to a heavies removal system that yields a liquid that is relatively concentrated in heavies and a diluent vapor that is relatively free of heavies. The liquid is passed to a heavies column, while the diluent vapor is passed to a diluent recycle chamber and then recycled to a slurry polymerization reactor without additional treatment to remove heavies.

RELATED APPLICATIONS

[0001] This application relates to and claims priority benefits fromU.S. Provisional Patent Application No. 60/411,254 entitled “Process andSystem for Removing Heavies From Diluent Recycled to a SlurryPolymerization Reactor,” filed Sep. 16, 2002, which is incorporated byreference herein.

FIELD OF THE INVENTION

[0002] The present invention generally relates to an olefinpolymerization system and process. More particularly, the presentinvention relates to an olefin polymerization system and process thatinclude an improved way of removing heavies from diluent and/orunreacted monomer withdrawn from and recycled to the polymerizationreactor.

BACKGROUND OF THE INVENTION

[0003] Olefin polymerization processes may be conducted under slurryconditions. Such polymerization processes may be carried out in a loopreactor in which monomers are polymerized to form a slurry of solidpolymer particles in a liquid medium. Portions of the slurry arewithdrawn from the loop reactor through take-offs devices such assettling legs and continuous take-offs.

[0004] The slurry is then processed to separate the liquid medium (forexample, a liquid diluent) from the solid polymer particles. Flashlineheaters and flash chambers have been used to vaporize the liquid diluentthrough a combination of temperature and pressure adjustments, therebyseparating the diluent from the solid polymer particles. Additionally,other types of equipment, such as purge columns, separating drums andcyclones, have been used for separation and other downstream treatment.

[0005] Efficient slurry polymerization processes typically recycle asmuch diluent as they reasonably can. That is, as the diluent isvaporized, diluent is processed for recycling back to the loop reactor.As the liquid diluent is vaporized at various stages of thepolymerization process after the slurry is withdrawn from the loopreactor, various compounds and contaminants may also be vaporized alongwith the diluent.

[0006] “Heavies” are liquid components heavier than the diluent, and areoften vaporized along with the diluent. During the recycling process, avapor stream containing the diluent and heavies may be condensed andreturned to the loop reactor as a recycle stream of the diluent andheavies. Heavies may concentrate or accumulate within the loop reactorand decrease reactor efficiency and/or cause damage within the loopreactor. For example, heavies buildup within the reactor may degradeproduct quality and/or distort reactor control making reactor problemsand blockage more likely.

[0007] Another recycling technique separates a slip stream comprisingdiluent and minor amounts of monomer from the bulk of the flashed fluidcomponents. The slip stream is subjected to olefin removal to give anessentially olefin-free stream for recycle to a catalyst preparationarea. The slip stream may be taken off before and/or after a diluentrecycle cooler. The slip stream(s) is then passed to a heavies removalcolumn. The heavies removal column removes heavies from the slip stream,which can then be used to prepare a catalyst mud or slurry. The bulk ofmonomer-containing recycle diluent is passed to a diluent recycle surge(storage) vessel, and a recycle stream is passed back to the reactor. Ifdesired, other purification steps such as water removal can be carriedout on this recycle stream.

[0008] An efficient system and process of recycling diluent in a slurrypolymerization process are desired. Further, an improved heavies removalsystem and process, which remove heavies from the diluent to be recycledto the reactor, are desired.

BRIEF SUMMARY OF THE INVENTION

[0009] As one aspect of the present invention, a process for recycling aliquid medium withdrawn from a slurry polymerization reactor isprovided. The method comprises separating a first vapor stream from aslurry comprising a liquid medium and solid polymer particles, whereinthe vapor stream comprises at least the medium and heavies. A firstportion of the first vapor stream is passed to a first condensing zone,and a first portion of the first vapor stream is condensed to form afirst liquid. A second portion of the first vapor stream is passed to acollection zone without substantial condensation. A second liquid and asecond vapor stream are separated in the collection zone. The secondliquid is passed to a heavies purification zone, and the medium isrecycled from the second vapor stream without fractionating to removeheavies.

[0010] As another aspect of the present invention, a recovery andpurification system for a liquid medium from a slurry polymerization isprovided. The system comprises a first fluid passage connected at oneend to a slurry polymerization reactor and connected at an opposite endto a flash gas separator, for separating a vapor stream comprising themedium from solid polymer particles. The system also comprises a vaporremoval line connected at a top portion of the flash gas separator, fortransporting the vapor stream from the flash gas separator. The vaporremoval line is connected to a first condenser and also to a vaporbypass line that provides a bypass around the condenser. A bypass valvecontrols the flow of vapor through the vapor bypass line. The output ofthe condenser and the opposite end of the bypass line are both fluidlyconnected to a liquid collection tank. A liquid delivery line is at abottom portion of the liquid collection tank, and a vapor recycle lineis at a top portion of the liquid collection tank. A second condenser isfluidly connected to the vapor recycle line, and a second fluid passageconnects the output of the second condenser to the slurry polymerizationreactor.

[0011] The present invention may include control apparatus or stepsassociated with the bypass valve. For example, the bypass valve can bein informational communication with at least one of a temperaturecontroller downstream of the liquid collection tank and a levelcontroller positioned within the liquid collection tank. The bypass lineis advantageous because it is difficult to operate a condenser toproduce only a small amount of liquids. The present invention is able toproduce a relatively small amount of liquids because the first vaporstream (which is the flash gas from the first flash chamber) is split,with a first portion passing through a condenser and a second portionbypassing the condenser. The flash gas passing through the condenser maybe entirely or nearly entirely condensed into liquid, or a small part(for example, about 1-10%) may be condensed, or some other proportions.

[0012] Downstream of the condenser, the hot flash gas is combined withthe cold liquid and mixed and allowed to come to an equilibrium (atleast with respect to temperature). After this mixing, the liquid thatremains is more concentrated in the heavier components. The desiredamount of liquid is set by the flow controller on the liquid line. Thelevel in the tank is controlled by adjusting the temperature controller,which in turn adjusts the bypass valve so that a sufficient amount ofvapor bypasses the condenser to generate enough liquid to satisfy theflow controller. To accomplish the desired amount of heavies removal,from about 0.1 to about 20% of the flash gas alternatively from about0.5% to about 10%, alternatively from about 1% to about 5%, will beconverted into liquids by the condenser. At times it may be desirable tocollect no liquids. At other times (for example, during resintransitions), it may be desirable to maximize liquids generation andcollect more than usual.

[0013] The present invention may also include a heavies column or otherheavies removal treatment at an opposite end of the heavies deliveryline. A flow controller and/or a pump can be disposed along the heaviesdelivery line.

[0014] A static mixer can be fluidly connected to and downstream fromthe first condenser, and the static mixer is upstream from and fluidlyconnected to the liquid collection tank. The static mixer is employed toquicken the formation of an equilibrium mixture after the hot flash gasand cold liquid are combined. It is desirable to have equilibriumconditions before entering the collection tank, at least a temperatureequilibrium. A recycle tank may be disposed along the diluent recycleline downstream of the flash gas condenser.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a representation of a slurry polymerization system whichincludes a novel heavies removal system.

[0016]FIG. 2 is a representation of the heavies condenser, static mixer,and liquid collection tank from the system of FIG. 1, as well as a novelcontrol scheme.

[0017]FIG. 3 is another representation of a slurry polymerization systemwhich includes a novel heavies removal system.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention provides a diluent recovery andpurification system and process for a slurry polymerization reactor,especially for a loop polymerization reactor. The system and processcomprise a flash chamber or flash gas separator, a vapor removal line, acondenser, and a liquid collection tank. The flash chamber or flash gasseparator receives a slurry withdrawn from a loop reactor. Diluent,unreacted monomer, and various heavies are separated from the solidpolymer particles as a first vapor stream. The vapor removal linereceives the first vapor stream and transfers a first portion to acondenser that condenses at least some of the first portion to form afirst liquid. It may be a relatively small amount portion, for example,about 2% of the first vapor stream that is condensed. The second portionof the first vapor stream bypasses the condenser but is thereaftercombined and mixed with the first liquid to form a combined stream. Thecombined stream is sent to a liquid collection tank. As the vapor andliquid mix in the mixing device and/or in the line to the liquidcollection tank, some of the liquid will be vaporized again, and themixture may come to an equilibrium temperature. The combined stream willbe cooled enough such that some liquid will be present. The liquid thatcollects in the liquid collection tank will have a higher concentrationof the heavier compounds than the vapor leaving the collection tank. Theliquid phase of an equilibrium mixture will be richer in the heaviercomponents than the vapor phase because the heavier components are lessvolatile.

[0019] The liquid containing a higher concentration of the heaviercomponents is passed from the liquid collection tank to a heaviesdelivery line, and the vapor is passed to a diluent recycle line. Thevapor may be passed to a second condenser and a recycle surge tank. Thevapor from the liquid collection tank can be condensed and recycled tothe slurry polymerization reactor without further treatment to removeheavies.

[0020] During the separating step, vapor comprising diluent, unreactedmonomers/comonomers and various inerts, both heavier and lighter thanthe primary diluent (for example, isobutane), are separated. Variousother compounds associated with the feedstocks and diluent feed, such asother compounds with 4 carbons (for example, n-butane) and compoundswith six carbons (for example, n-hexane), are inerts, which do notpolymerize and function as a diluent along with isobutane. Heaviesinclude additional compounds with six carbons along with any heaviercompounds that may be produced in the reactor (such as oligomers). Aportion of the vapor stream is then passed to a condenser in a heaviesremoval zone. The condenser may only condense a small portion of thestream or it may condense most or essentially all of the first portion.The liquid and vapor are then combined, which allows them to form acombined stream. The combined stream is passed to a liquid collectionzone, and liquid and vapor are separated in the collection zone, afterwhich the diluent vapor is passed to a diluent recycle line and theliquid is passed into a heavies delivery line.

[0021] The diluent vapor separated from the liquid (that is, diluentvapor exiting from a top portion of the liquid collection tank) issuitable for recycling to the polymerization reactor. The diluent vaporis passed from the diluent recycle line to a diluent recycle tank. Thediluent vapor is condensed by heat exchange. The diluent can then berecycled to the polymerization zone without having to remove additionalheavies. Further, the liquid is passed from the heavies delivery line toa heavies column.

[0022] The present invention is applicable to any olefin polymerizationin a loop reactor utilizing a liquid medium so as to produce a fluidslurry of polymer solids in the liquid medium. Suitable olefin monomersinclude 1-olefins having up to 8 carbon atoms per molecule and nobranching nearer the double bond than the 4-position. The presentinvention is particularly suitable for the homopolymerization ofethylene and copolymerization of ethylene and a higher 1-olefin such asbutene, 1-pentene, 1-hexene, 1-octene or 1-decene.

[0023] Suitable diluents for use as the liquid medium are well known inthe art and include hydrocarbons, which are inert and liquid underreaction conditions. Suitable hydrocarbons include isobutane, propane,n-pentane, i-pentane, neopentane and n-hexane, with isobutane beingespecially preferred. Additional details regarding loop reactorapparatus and polymerization processes may be found in U.S. Pat. Nos.4,674,290; 5,183,866; 5,455,314; 5,565,174; 5,624,877; 6,005,061;6,045,661; 6,051,631; 6,114,501; 6,262,191; and 6,420,497, which areincorporated by reference herein.

[0024] Additionally, the present techniques for controlling heavies maybe employed where the monomer is the liquid medium for thepolymerization. For example, the present techniques may be used for thepolymerization of propylene where propylene is the liquid medium and aninert diluent is not present in any substantial amount. A diluent maystill be used for the catalyst. For illustration, but not as alimitation, the present invention will be described in connection with apolyethylene process using an inert diluent as the liquid medium, but itis to be understood that the present invention may also be employedwhere the monomer is used as the liquid medium and would take the placeof the diluent in the following descriptions.

[0025]FIG. 1 is a representation of a slurry polymerization system 10according to the present invention. The slurry polymerization system 10includes a catalyst feed tank 12 (such as a catalyst mudpot or anagitated tank) containing a mixture of catalyst and diluent. Catalystand diluent from the catalyst tank 12 are pumped into a loop reactor 14.FIG. 1 also shows flashline heaters 16, a flash gas separator (alsodescribed as an intermediate pressure flash chamber) 18, a heaviesremoval system 20, a fluff chamber 22, a purge column 24, a recycletreater 26, an isobutane/nitrogen recovery unit (INRU) 28, a recycletank 30, a heavies column 32, a lights column 34, an olefin free surgeunit 36, and a series of conduits, pumps and condensers. For the sake ofsimplicity, some components unrelated to the claimed systems andprocesses and/or that are related to production or routine details, suchas intervening valves and connecting lines/conduits of the slurrypolymerization system 10, are not shown.

[0026] Catalyst or catalyst and diluent from the catalyst feed tank 12passes through conduit 40 and is pumped into the loop reactor 14 by pump42. Suitable catalysts are well known in the art. For example, chromiumoxide on a support such as silica may be used, as disclosed in U.S. Pat.No. 2,825,721, issued to Hogan and Banks, which is herein incorporatedby reference in its entirety. Additionally, other catalysts well knownin the art (for example, Ziegler catalysts, metallocene catalysts) mayalso be used for olefin polymerization.

[0027] The slurry polymerization system 10 also includes a monomer feed38. A monomer such as ethylene, for example, is introduced into the loopreactor 14. Additional conduits and lines may supply monomer and/ordiluent into the loop reactor 14 or join in a line 46 for introducing acombined stream of fresh ethylene and diluent recycle as shown. Monomer,diluent, catalyst and any other feed materials may be introduced intothe loop reactor 14 at one or several points.

[0028] As shown in FIG. 1, the loop reactor 14 may comprise a pluralityof vertical members 48 formed integrally with horizontal members 50 (orcurved connecting members). In order to maximize heat transfer withinthe reactor 14, the distance between vertical members 48 is preferablyminimized. Thus, the horizontal members 50 may be of minimal length.Alternatively, the horizontal members 50 may be eliminated such that thevertical members 48 are connected through curved connecting members.Alternatively, the loop reactor may be substantially horizontal and nothave any vertical members. The longer members 48 preferably have heatexchange jackets 49. The vertical members 48 and horizontal members 50(or curved connecting members) define a loop reaction zone. The loopreaction zone may include more or fewer vertical members 48 andcorresponding horizontal members 50 as that shown in FIG. 1. Further,the loop reactor may be oriented vertically or horizontally (forexample, by rotating reactor 14 in FIG. 1 by 90 degrees) or may beentirely horizontal, with no vertical members. The connecting members 50may be any shape or form that connects the vertical segments 48 andallows fluid to flow therebetween.

[0029] An impeller is located in the loop reactor 14 to circulate theslurry. The impeller is driven by a motor 47. The impeller is located inthe interior of the loop reaction zone defined by the vertical members48 and the horizontal members 50. The impeller 47 is operable tocirculate fluid slurry, comprising liquid diluent and solid olefinpolymer particles, through the loop reactor 14.

[0030] An intermediate product slurry, comprising the liquid medium andsolid polymer particles, is withdrawn, or taken off, from the loopreactor 14 by way of the continuous take-off devices 52. A continuoustake-off device 52 is disclosed in U.S. Pat. No. 6,239,235, which isincorporated by reference herein. The slurry polymerization system 10may include more or fewer than the continuous take-off devices 52 (andcorresponding conduits and flashline heaters 16) shown in FIG. 1. Forexample, one, two, three or more continuous take-off devices 52 may beused. Further, the continuous take-off devices 52 may be tangentiallypositioned on curved members. The continuous take-off devices 52 may bepositioned anywhere on the loop reactor 14. Alternatively, settling legsmay be used in conjunction with, or instead of, the continuous take-offdevices 52.

[0031] As fluid slurry is withdrawn from the loop reactor 14 asintermediate product slurry, the intermediate product slurry is passedfrom the continuous take off-device 52 to a flashline 54, which forms afirst fluid passage for the withdrawn portion of the slurry todownstream processing apparatus. The flashline 54 may be provided with aflashline heater 16 surrounding at least a portion of the flashline 54.The flashline heater 16 contains a heated fluid (for example, steam)that provides indirect heating to the contents of the flashline 54, suchthat intermediate product slurry that passes through the flashline 54 isheated. Preferably, the intermediate product slurry is heated such thatat least a majority of the liquid diluent is vaporized, thereby yieldingdiluent vapor and a post-flashline slurry. The post-flashline slurrycomprises the solid polymer particles and a reduced amount of liquiddiluent (as compared to the intermediate product slurry). Alternatively,the post-flashline slurry may be heated such that less than a majorityof the liquid diluent is vaporized. Preferably, the intermediate productslurry is heated in the flashline 54 such that, as it enters the flashgas separator 18, essentially all of the liquid diluent has beenvaporized (“flashed”) within the flashline 54. The “flashing” tends tooccur as the intermediate product slurry passes across the continuoustake-off device 52 and the flashline 54. Preferably, the intermediateproduct slurry is heated in the flashline 54 to fully vaporize thediluent liquids so that the solids and vapors that discharge into theflash gas separator 18 are free of liquids.

[0032] In some systems utilizing a flashline heater 16, some or all ofthe diluent (or other liquid medium) will flash in flashline 54 prior tointroduction to the flash gas separator 18, which may be termed a “flashchamber” or an “intermediate pressure flash chamber.” These terms stillare frequently used for the tank that follows the flashline, wherevaporized diluent separates from polymer solids. “Flash tank” or “flashchamber” are still used even though there may be little or no flashingin the flash tank if all or substantially all of the diluent isvaporized in the flashline. In current designs that have the flashlinesdischarging at higher pressures and without downstream drying devices,it is intended to design the flashlines so there is little or nopressure drop on entering the flash tank, with essentially all of theliquids vaporized prior to entering the vessel.

[0033] After the intermediate product slurry passes through theflashline 54, the resulting post-flashline fluff and vaporized diluentis passed into the flash gas separator tank or chamber 18. The flash gasseparator 18 preferably is at an intermediate pressure, lower than thepressure in the reactor but higher than the pressure of downstreamapparatus. Preferably the flash gas separator 18 is at a pressurewhereby the separated flash gas (or a majority or other large portion)may be condensed by heat exchange without compression. In the flash gasseparator 18, most of the diluent, unreacted monomer, and heavies form avapor stream (flash gas) that rises toward a top portion of the flashgas separator 18, while “fluff” falls to a bottom portion of the flashgas separator 18. The fluff comprises solid polymer, which may havetrace or other small amounts of diluent entrained therein.

[0034] The fluff may pass to a second flash gas separator (for example,a low pressure flash chamber). A two-stage flash system is disclosed inU.S. Pat. No. 4,424,341, which is incorporated by reference herein.Alternatively, the fluff may pass from a bottom portion of the firstflash chamber to the purge column 24. (The purge column 24 may followthe second flash chamber instead). Alternatively, the fluff may bepassed through a conveyor dryer and then to a purge column as describedin U.S. Pat. No. 4,501,885, which is incorporated by reference herein.Entrained diluent within the polymer particles is separated from thefluff in the purge column 24 by passing nitrogen gas through the solidpolymer particles (the fluff). The nitrogen extracts entrained diluentand/or liquid diluent, thereby leaving solid polymer essentially free ofentrained diluent. The solid polymer is then deposited, collected,ejected or otherwise withdrawn from a bottom portion of the purge column24.

[0035] The nitrogen and extracted diluent are then passed out a topportion of the purge column 24 to an isobutane/nitrogen recovery unit(INRU) 28. The INRU processes the vapors removed from the fluff in thepurge column. The INRU separates the nitrogen from the diluent vapors bycondensing the diluent vapors into liquids. In one sense the INRUfunctions somewhat like the heavies removal system by condensing arelatively small amount of the flash gas and sending a liquid to theheavies column. The INRU 28 separates nitrogen from diluent and otherhydrocarbons. The nitrogen is then passed back to the purge column 24through a nitrogen return line 60. The separated diluent and otherhydrocarbons may be returned to the recycle tank 30 and ultimatelyreturned to the loop reactor 14. Alternatively, some or all of theliquids from the INRU 28 may be sent to the heavies column 32. The INRUis designed to produce a liquid product which contains substantially allthe hydrocarbons (but not the solid polymer particles) removed in thepurge column.

[0036] The vapor stream which has been vaporized either in the flashline54 and/or in the flash gas separator 18, which is also referred toherein as the first vapor stream, is passed to the heavies removalsystem 20 by way of vapor removal line 56. The vapor removal line 56 mayinclude a series of filters and components, such as bag filters, forfiltering fine polymer particles from the vapor stream in order toprevent the fines from entering the heavies removal system 20. Forexample, for a suitable polyethylene process, the vapor stream isprimarily isobutane diluent, but the vapor stream diluent also containsheavies, such as 1-hexene co-monomers and other hydrocarbons having sixor more carbon atoms. The vapor stream may also contain lighterhydrocarbons such as ethane and ethylene.

[0037]FIG. 2 is a representation of an exemplary heavies removal system20. The heavies removal system 20 is fluidly connected to, andcommunicates with, a top portion of the flash gas separator 18 throughthe vapor removal line 56. In the equipment shown in FIG. 2, the heaviesremoval system 20 includes a first condenser 62, a bypass line 63, abypass valve 64, a static mixer 66, a liquid collection tank 68, atemperature controller 70, a level controller 72, a pump 74, a flowcontroller 76 and a flow control valve 78.

[0038] The heavies removal system 20 is designed and/or set so as tocondense a small amount of liquid from the first vapor stream. Afraction of flash gas (in other words, a first portion of the firstvapor stream) is passed through the condenser 62, which condenses someof that fraction into a liquid. When the liquid from condenser 62contacts the relatively hot flash gas that has bypassed the condenser 62(in other words, a second portion of the first vapor stream), some ofthe liquid again vaporizes, leaving a residual amount of liquid and alarger amount of flash gas. The liquid and flash gas are passed to theliquid collection tank 68. A static mixer 66 can be used to assureproper contact, rapid revaporization and equilibrium conditions in theliquid collection tank 68. The rate of flow may be set on flowcontroller 76. The level in the liquid collection tank 68 is determinedby the amount of liquid generated by the condenser 62. If the liquidlevel in the collection tank 68 rises, the temperature controller 70adjusts to a higher temperature to decrease the amount of liquidsgenerated by the condenser 62. Conversely, if the level within theliquid collection tank 68 decreases, the level controller 72 acts todecrease the temperature set point on the temperature controller 70,thereby directing more flow through the condenser 62 to generate moreliquids. Consequently, the level controller adjusts the temperaturecontroller to generate an amount of liquids that matches the desiredamount of liquids as set by the flow controller. This provides anefficient and relatively easy way to control the amount of liquidcondensed from the first vapor stream.

[0039] As seen in FIG. 1, the condenser 62 of the heavies removal system20 is in addition to flash gas condenser 84. The heavies removal system20 will generally be used to condense a relatively small amount of thetotal vapor from vapor removal line (in other words, of the first vaporstream). For example, the heavies removal system 20 may condense (inother words, form a liquid from) as little as about 1% or less of thetotal vapor stream from the first flash chamber. One may desire to set amaximum for the amount of the first vapor stream condensed by the firstcondenser. For example, the heavies removal system 20 may be set tocondense at most about 10%, alternatively at most about 5%,alternatively at most about 4%, alternatively at most about 3% of thefirst vapor stream. Condenser 62 may condense from about 0.1 to about15% of the flash gas. The amount of liquid condensed by condenser 62 maybe up to 5 times greater than the liquid generated by the heaviesremoval system 20.

[0040] The heavies removal system 20 produces liquid with a higherpercentage of the heavier components than is found in the vapor stream.For example, the liquid in the collection tank 68 may contain at leastabout 1%, alternatively at least about 5%, alternatively at least about10%, of the heavies in the vapor stream. In particular, the heaviesremoval system is expected to be particularly effective in removing mostoligomers, for example, at least about 2%, alternatively at least about10%, alternatively about 20% of the oligomers in the vapor stream. Evenif the heavies removal system does not remove all the heavies from theliquid medium to be recycled to the polymerization reactor, it stillprovides an important benefit by preventing an excessive buildup ofheavies in the recycled medium.

[0041] The liquid concentrated in heavies may be referred to as aheavies liquid. The heavies liquid is routed to a heavies column wherethe heavier components can be removed from the process. Thus theconcentration of heavier components in the flash gas is decreased fromlevels that would otherwise exist.

[0042] The present process and system result in the generation of arecycle diluent stream having fewer heavies than would otherwise buildup in the recycle diluent stream. This system can work in conjunctionwith a direct recycle process, allowing most of the diluent to becondensed and recycled directly back to the reactor without passingthrough fractionation columns that remove heavier and lightercomponents. In the direct recycle process it is desirable that only afraction of the total recycle diluent is sent to fractionation whereheavier components can be rejected and lights can be removed andolefin-free diluent generated. With only a fraction of the recyclediluent feeding fractionation, the fractionation columns can berelatively small.

[0043] As shown in FIG. 2, the condenser 62 is in fluid connection withthe vapor removal line 56 and is upstream from the static mixer 66. Thebypass valve 64 is interposed within the bypass line 63 such that thebypass valve 64 may selectively open and close the bypass line 63,thereby controlling the flow of vapor through the bypass line 63. Theliquid collection tank 68 is downstream from the static mixer 66. Adiluent recycle line 80 is in fluid communication with the liquidcollection tank 68 and connects the liquid collection tank 68 to arecycle tank 30. The bypass valve 64 may be in informationalcommunication with the temperature controller 70, which is interposedwithin the diluent recycle line 70. Additionally, or alternatively, thebypass valve may be in informational communication with the levelcontroller 72, which is positioned within the liquid collection tank 68.The temperature controller 70 and the level controller 72 may beassisted by or used in conjunction with a central processing unit orother logic unit or main controller. The temperature controller 70 andthe level controller 72 relay signals to the bypass valve 64 to close oropen the bypass line 63, depending on temperature and level conditionswithin the heavies removal system 20. This control scheme permitsautomatic control to obtain the desired amount of liquid in the liquidcollection tank 68.

[0044] The bottom portion of the liquid collection tank 68 is in fluidcommunication with a heavies delivery line 82, which receives a liquidconcentrated in heavies from the liquid collection tank 68. A pump 74may be positioned in the heavies delivery line 82, which providessufficient force to move the liquid concentrated in heavies through theheavies delivery line 82 to the heavies column 32. A flow controller 76,which may be assisted by or used in conjunction with a centralprocessing unit or logic unit or a main controller, is positioneddownstream from the pump 74 and is in informational communication withthe flow control valve 78, which may selectively open and close theheavies delivery line 82. The flow control valve 78 may be operated inconjunction with the flow controller 76. Depending on the rate of flowof heavies through the heavies delivery line 82, the flow controller 76may send a command signal to the flow control valve 78 to open or closethe heavies delivery line 82.

[0045] The heavies removal system 20 is used to produce a liquids streamrelatively rich in heavier components. Once the heavies or a portion ofheavies are removed, the diluent may be recycled and returned to theloop reactor 14. The heavies removal system 20 does not necessarilyremove all heavies. Rather, the heavies removal system 20 may onlyremove a small portion of the heavies in order to limit potentialbuild-up of heavies to relatively low levels. In particular, the heaviesremoval system can limit the build-up of oligomers.

[0046] The heavies removal system 20 is configured to produce liquidsrelatively concentrated in heavies that were contained within the firstvapor stream. It is contemplated that at least some of the heavies fromthe first vapor stream, or a major portion thereof, will be passed tothe heavies delivery line 82. For example, about 98% of the first vaporstream may remain as a vapor as a result of bypassing the firstcondenser, in other words only a small amount of liquids are generated.A portion of the first vapor stream is sent to the condenser where allor most of that portion is converted to liquid. This colder liquid iscombined with the hotter gases that bypassed the condenser and thisliquid and vapor mixture is sent to the static mixer 66. The staticmixer ensures good mixing of the liquids and vapors so thatsubstantially equilibrium conditions can quickly be generated.

[0047] Substantially equilibrium conditions means that the liquid andvapor in the liquid collection tank come to substantially the sametemperature with some of the liquids vaporizing to accomplish this. Thegas may then be passed to collection tank where the liquids and vaporsinitially intermingle but also are separated. As flash gas condenses andre-evaporates, heavies are left behind in the liquid at the bottom ofthe collection tank.

[0048] The amount of the first vapor stream that is diverted into thecondenser 62 is controlled to generate a desired amount of liquids inthe liquid collection tank 68. The bypassed hot vapor andcondenser-cooled liquid from the condenser 62 are then mixed by thestatic mixer 66. The temperature controller 70 may signal the bypassvalve 64 to open or close to a desired extent, depending on thetemperature of the diluent vapor in the diluent recycle line 80.

[0049] After the flash gas vapor and condensed liquid are mixed togetherto form a mixture, the mixture is passed into the liquid collection tank68. The liquid collection tank 68 separates the liquids containing ahigher concentration of the heavier components from the diluent vapor.It is desirable to reduce or avoid pressure or temperature gradients inthe liquid collection tank.

[0050] The temperature controller 70 can be periodically or occasionallyreset by the level controller on the collection tank. If the level inthe collection tank drops below the level setpoint, the controller sendsa signal to lower the setpoint of the temperature controller which inturn sends a signal to close the bypass valve to a desired extent andforce more flow through the condenser to generate more liquids.Conversely, if the level in the tank is above the desired setpoint, thelevel controller sends a signal to raise the setpoint of the temperaturecontroller which in turns sends a signal to open the bypass valves to adesired extent which decreases the flow through the condenser whichreduces the amount of liquids formed. When the setpoint of the flowcontroller is changed, the flow control valve opens or closes to producethe desired flow, and this change of flow is reflected as a change inlevel in the tank, and the level controller reacts to change the amountof liquids condensed.

[0051] The liquid concentrated in heavies is passed from the liquidcollection tank 68 through the heavies delivery line 82 and pumpedtowards the heavies column 32 by the pump 74. The flow controller 76monitors and controls the flow of the liquid through the heaviesdelivery line 82 by signaling the flow control valve 78 to open or closethe flow control valve 78, depending on the desired amount of liquids tobe generated. For example, if the operators want to create 1500 kg/hrliquids, it is desirable that that number can be entered as a setpointto the flow controller. The liquids are ultimately delivered to theheavies column 32.

[0052] Referring again to FIG. 1, the heavies column 32 separates theliquid medium and lighter components from the heavies. The diluent (inother words, the liquid medium) extracted in the heavies column 32 isthen passed to a lights column 34, where lights are removed, therebyyielding essentially or substantially pure diluent, which is then passedthrough the olefin-free surge 36 and pumped to the catalyst feed tank 12or to the recycle tank 30, and it is then pumped to the loop reactor 14.

[0053] The diluent vapor within the diluent recycle line 80 is passedthrough a flash gas condenser 84, which may condense, or liquefy, thediluent. Some diluent with a relatively high concentration of lightercomponents may remain as vapor. The stream is passed to the recycle tank30, which serves as a vapor-liquid separation drum and the liquids arepassed directly back to the loop reactor 14 through the recycle treater26. The vapor which has a higher relative concentration of the lightercomponents is passed to fractionation where the light components arerejected, and 1-hexene and hexanes are recovered or rejected, octane andheavier compounds are rejected, and diluent (usually isobutane) isrecovered free of olefins.

[0054]FIG. 3 is a schematic representation of another slurrypolymerization system 11, which includes a novel heavies removal system.As shown in FIG. 3, the lights removal line 86 may be a vapor line thatenters through the side of the heavies column 32. Since vapor line 86will contain small amounts of hexane, it is desirable to have it enterthe heavies column because all hexanes must be excluded from the lightscolumn. Also, there is a liquid product line 90 extending from the INRU28. The INRU is designed to recover both nitrogen and hydrocarbons(diluent) as liquid. The liquid product line 90 may be split with onebranch connecting to the heavies column 32 and another branch connectingto the recycle tank 30. In this way, liquids may be directed to theheavies column 32 and/or to the recycle tank 30. Also, the olefin-freeproduct that is passed from the olefin-free surge unit 36 may be pumpedback to the recycle tank 30 in addition to the catalyst mix tank 12.

[0055] Thus, the present invention provides a more efficient system andprocess of recycling the liquid medium in a slurry polymerizationprocess. The present invention provides an improved heavies removalsystem and process, which removes a greater amount of heavies, ascompared to prior systems, from the liquid medium that is to be recycledto the reactor. A direct recycle system is provided where most or all ofthe liquid in the reactor effluent (diluent and other hydrocarbons) isseparated from the fluff, condensed and recycled back to the reactorwithout passing through purification (fractionation) steps. Othersystems pass substantially all of the diluent vapor (reactor effluent)through fractionation (purification steps) where all of the heaviercompounds can be separated. In a direct recycle system there is a desireto reject heavier components as well as lighter components so they donot concentrate in the process and harm the reactor or product. So inthe direct recycle process some recycle diluent (in other words, reactoreffluent hydrocarbons) are still processed in a small purification(fractionation) section to generate some pure (olefin-free) diluent foruse as catalyst diluent, and to reject some heavy and light componentsto prevent abnormal build up or concentration. By fractionating only asmall portion of the diluent recycle, the fractionators can be smaller.The direct recycle fractionators can be fed from a recycle liquid slipstream or by INRU liquid product but by creating liquids with thepresent heavies removal system, heavies concentration is controlledbetter, in other words for the same fractionation feed rate, heaviesconcentration in the recycle is lower.

[0056] The present system and process described may also feedfractionation (heavies and lights columns) when the INRU is notoperational so that olefin-free diluent may be generated to slurry thecatalyst.

[0057] While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from its scope.Therefore, it is intended that the invention not be limited to theparticular embodiment disclosed, but that the invention will include allembodiments falling within the scope of the appended claims.

That which is claimed is:
 1. A recovery and purification system for aliquid medium from a slurry polymerization, the system comprising: afirst fluid passage connected at one end to a slurry polymerizationreactor; a flash gas separator connected to an opposite end of the firstfluid passage, for separating a vapor stream comprising solid polymerparticles in a liquid medium; a vapor removal line connected at a topportion of the flash gas separator, for transporting the vapor streamfrom the flash gas separator; a first condenser downstream of the vaporremoval line; a collection tank downstream of the first condenser; avapor bypass line providing a bypass for a portion of the vapor streamaround the condenser to the collection tank; a bypass valve forcontrolling the flow of vapor through the vapor bypass line; a liquiddelivery line at a bottom portion of the collection tank; a vaporrecycle line at a top portion of the collection tank; a second condenserfluidly connected to the vapor recycle line; and a second fluid passageconnecting the output of the second condenser to the slurrypolymerization reactor.
 2. The recovery and purification system of claim1, wherein the bypass valve is in informational communication with atleast one of a temperature controller downstream of the collection tankand a level controller positioned within the collection tank.
 3. Therecovery and purification system of claim 2, wherein the levelcontroller is in informational communication with the temperaturecontroller, and the temperature controller is in informationalcommunication with the bypass valve.
 4. The recovery and purificationsystem of claim 1 further comprising a heavies column connected at anopposite end of the liquid delivery line.
 5. The recovery andpurification system of claim 4 further comprising a flow controllerdisposed along the liquid delivery line.
 6. The recovery andpurification system of claim 4 further comprising a pump disposed alongthe liquid delivery line for propelling the liquid into the heaviescolumn.
 7. The recovery and purification system of claim 1 furthercomprising a static mixer fluidly connected to and downstream from thefirst condenser, and the static mixer is upstream from and fluidlyconnected to the collection tank.
 8. The recovery and purificationsystem of claim 1, further comprising a recycle tank disposed along thediluent recycle line downstream of the second condenser.
 9. The recoveryand purification system of claim 1 further comprising a purge columnfluidly connected to receive solid polymer particles from the flash gasseparator.
 10. A process for recycling a liquid medium withdrawn from aslurry polymerization reactor, the method comprising: separating a firstvapor stream from a slurry comprising a liquid medium and solid polymerparticles, wherein the vapor stream comprises at least the medium andheavies; condensing a first portion of the first vapor stream in a firstcondensing zone to form a first liquid; passing a second portion of thefirst vapor stream to a collection zone without passing through thefirst condensing zone; separating a second liquid and a second vaporstream in the liquid collection zone; passing the second liquid to aheavies purification zone; and recycling the condensed medium to theslurry polymerization reactor without fractionating to remove heavies.11. The recycling process of claim 10 further comprising the step ofmixing together the first liquid and the second portion of the firstvapor stream before passing to the collection zone.
 12. The recyclingprocess of claim 10 wherein the separating step yields solid polymerfluff having entrained diluent in addition to the vapor stream, and themethod further comprises: passing the solid polymer fluff from theflashing step to a purge zone wherein the purge zone extracts entraineddiluent from the solid polymer fluff.
 13. The recycling process of claim10 wherein at least about 1% of the heavies in the first vapor streamare condensed into the second liquid.
 14. The recycling process of claim10 wherein at least about 5% of the heavies in the first vapor streamare condensed into the second liquid.
 15. The recycling process of claim10 wherein at least about 10% of the heavies in the first vapor streamare condensed into the second liquid.
 16. The recycling process of claim10 wherein at least about 2% of the oligomers in the first vapor streamare condensed into the second liquid.
 17. The recycling process of claim10 wherein at least about 10% of the oligomers in the first vapor streamare condensed into the second liquid.
 18. The recycling process of claim10 wherein at least about 20% of the oligomers in the first vapor streamare condensed into the second liquid.
 19. The recycling process of claim10 further comprising measuring the level of liquid in the liquidcollection zone; and controlling the condensing of the vapor streambased on the measured liquid level.
 20. A process for operating a slurrypolymerization system, the process comprising the steps of: forming aslurry comprising solid polyolefin particles in a liquid medium in apolymerization reaction zone; withdrawing a portion of the slurry fromthe polymerization reaction zone; vaporizing the liquid medium to form afirst vapor stream comprising at least the medium and heavies;separating the first vapor stream from the solid polyolefin particles;passing a first portion of the first vapor stream to a condensation zonewhereby a first liquid stream is formed; passing a second portion of thevapor stream to a collection zone without passing through thecondensation zone; collecting the liquid stream and the second portionin the liquid collection zone; separating a second vapor stream from asecond liquid stream in the liquid collection zone; condensing andrecycling the second vapor stream back into the polymerization reactionzone without further heavies removal treatment; and passing a secondliquid stream to heavies removal treatment.
 21. The operating process ofclaim 20 wherein the liquid medium is an inert diluent, and the solidpolyolefin particles are polyethylene.
 22. The operating process ofclaim 20 wherein the liquid medium is propylene, and the solidpolyolefin particles are polypropylene.
 23. The operating process ofclaim 20 further comprising passing the solid polyolefin particles to apurging step, where entrained diluent is extracted from the solidpolyolefin particles.
 24. The operating process of claim 20 furthercomprising measuring the level of liquid in the liquid collection zoneand controlling the condensing of the first vapor stream based on themeasured liquid level.
 25. The operating process of claim 20 furthercomprising passing the second liquid stream from the collection zone toa heavies fractionation column.
 26. The operating process of claim 20further comprising the steps of: measuring the temperature of the vaporin the liquid collection zone; and adjusting the amount of the firstvapor stream being passed to the condensation zone in response to themeasured temperature.
 27. The operating process of claim 20 wherein theliquid removed from the collection zone contains at least about 1% ofthe heavies in the first vapor stream.
 28. The operating process ofclaim 20 wherein the liquid removed from the collection zone contains atleast about 5% of the heavies in the first vapor stream.
 29. Theoperating process of claim 20 wherein the liquid removed from thecollection zone contains at least about 10% of the heavies in the firstvapor stream.
 30. The operating process of claim 20 wherein the liquidremoved from the collection zone contains at least about 2% of theoligomers in the first vapor stream.
 31. The operating process of claim20 wherein the liquid removed from the collection zone contains at leastabout 10% of the heavies in the first vapor stream.
 32. The operatingprocess of claim 20 wherein the liquid removed from the collection zonecontains at least about 20% of the heavies in the first vapor stream.33. The operating process of claim 20 wherein essentially all theheavies removed from the liquid collection zone are removed in liquidform.
 34. The operating process of claim 20 wherein the second liquidstream is held in the liquid collection zone for a time sufficient toallow heavies from the vapor to transfer to the liquid.